You can access the 'Webcam Button' through the 'Toolbar'.

ScreenCamera has the ability to mix your desktop screen with your webcam, and it also has the ability to show only your desktop screen or your webcam on the video output. When you click the webcam button you include or exclude the webcam from the video output.

Here you see an example of your desktop screen alone and your desktop screen mixed with your webcam as a secondary video source: